This invention relates to fasteners for connecting adjacent structural members and, more particularly, to a rivet having a deformable head for providing uniform fill of rivet holes in the structural members.
Rivets are used in various industries as an inexpensive fastener. The rivets generally include a head and a shank which are inserted into aligned holes of adjacent structural members and deformed to permanently join the structures. In the aerospace industry rivets are used in many applications, such as attaching aircraft skin to wing structure. Riveted structures are often selected for their ease of installation and enhancing fatigue characteristics of the structure. These characteristics, however, are dependent on the hole-filling capability of the rivet. Rivets presently available do not provide both sufficient radial enlargement of the head, and deformation and expansion of the head toward a head to shank junction. This prevents the formation of compressive residual stress in the surrounding structure required to provide a satisfactory fatigue life of the structural joint.
Conventional rivets have a preformed head to conform to the shape of the bore in the structure, thus little cold working of the head takes places during installation of the rivet. The preformed head prevents expansion of the rivet at the head to shank junction which results in lack of hole fill in this critical area. Improper hole fill may result in relative movement between structural components which may initiate fatigue cracks. Furthermore, if adequate hole fill is not achieved, fluids and sealants can enter gaps between the rivet and structure and may get entrapped and cause corrosion and weakening of the joint.
A prior art rivet R is shown in FIG. 1 inserted into two structural members M prior to upsetting of the rivet. The rivet has a head H which generally conforms to the shape of a countersink C in the structural member. During upsetting of the rivet R, the head H is only slightly deformed and undergoes a small amount of radial expansion to fill the countersink C. Since the head H has previously been formed to its near final shape, the preformed head inhibits expansion of the rivet R in a head to shank junction J.
Moreover, the rivet shown in FIG. 1 must be precisely sized to provide sufficient volume to adequately fill the countersink without protruding beyond the surface of the structure. The rivet head must be as flush as possible for aerodynamic surfaces of high performance aircraft since surface irregularity can degrade performance due to aerodynamic drag. If the volume of the head is smaller than the countersink, the rivet will not adequately fill the opening and leave gaps between the rivet and structure, if the volume of the head is greater than the countersink, the rivet will extend beyond the surface of the structure. The structure or rivet will then have to be machined to provide a flush surface. This extra machining step is time consuming and increases the manufacturing costs of the structure and aircraft.
In order to eliminate this final machining, precision drilled structures and precision machined rivets have been developed. This significantly increases the cost of the rivet and machining of the structure. These close tolerance requirements also result in a high rejection rate of the structural joint.